The present invention relates to pressure-sensitive adhesive bonding tapes, and more particularly to a bonding tape having a layer of a primary pressure-sensitive adhesive material coated with a film of a secondary pressure-sensitive adhesive material.
Pressure sensitive adhesive tapes are commonly employed in adhesive fastening in a wide variety of application. However, in situations were a bond must be made to or between low energy surfaces, adequate adhesion is difficult to achieve. A low energy surface may be defined for purposes of this specification as a surface having a critical surface tension low enough so that there will not be sufficient wetting by an adhesive. An example of adhesive bonding between low energy surfaces is the bonding of an automobile windshield to a car body. The surfaces to be joined are glass and painted metal (especially the newer acrylic high solids enamel paints), both low energy surfaces.
The principal problem in obtaining adequate adhesion to low energy substrates is poor surface to surface contact between adhesive and substrate because the adhesive cannot properly spread and wet the substrate. This may result in adhesive failure during use. Moreover, the difficulties in achieving good adhesion to a low energy surface are increased when excess moisture is present and/or during application of repeated cyclic stresses such as vibrations. Both conditions are common in the bonding of windshields to automobile bodies.
One method practiced by the prior art in attempting to overcome poor adhesion to low energy surfaces is to apply a primer to the low energy surface prior to bonding. The primer is usually applied as a liquid and possesses a surface tension low enough to promote wetting of the substrate. Upon solvent release, or other suitable mechanisms, the primer sets up to form a continuous film over the surface of the substrate. With the film of primer in place, suitable bonding of the adhesive to the substrate can be achieved. Although the use of primers overcomes the poor adhesion characteristics of pressure-sensitive adhesive tapes to low energy surfaces, the need to apply a primer prior to bonding increases the time, costs, and labor to perform such operations. Significant time, labor and material costs are required to apply and then cure or dry primer layers on one or more of the low energy substrate surfaces to be bonded.
The use of energy-absorbing bonding tapes coated with pressure-sensitive adhesives, such as the energy-absorbing bonding tapes disclosed in commonly-assigned U.S. Pat. Nos. 3,896,245 and 4,061,805, is also known for adhering automotive trim strips and the like to painted metal substrates. However, where it is desired to bond two low energy surfaces together, it has been found that with the pressure-sensitive adhesives heretofore utilized with such bonding tapes that bonding performance is often inadequate, especially where the bond is subjected to moist conditions and/or cyclic stresses.
Accordingly, the need exists in the art for a pressure-sensitive adhesive bonding tape which overcomes the problems of prior art tapes and yet achieves adequate bonding without the need for the application of a separate layer or layers of primer to the substrate(s) to be bonded. By achieving adequate adhesion without the use of primers, the user would realize both time and cost savings.